function s = extractPartQF(q, top, left, qf_b, qf_s)
    % q is 16x16 matrix, top, left is the position of DCT Coef
    bq0 = idct2(q(1:8,1:8).*qf_b,8,8);
    bq1 = idct2(q(1:8,9:16).*qf_b,8,8);
    bq2 = idct2(q(9:16,1:8).*qf_b,8,8);
    bq3 = idct2(q(9:16,9:16).*qf_b,8,8);
    bq = [bq0,bq1;bq2,bq3]+128;
    sq = bq(top:top+7,left:left+7);
    s = dct2((sq-128), 8, 8)./qf_s;
end
